Acoustic liners are used in fluid handling ducts to attenuate undesirable noise associated with a stream of fluid flowing through the duct. Examples of such ducts include the inlet and exhaust system ducts of gas turbine engines. A typical acoustic liner includes a back sheet, a face sheet spaced from the back sheet, and a series of walls that extend between the face sheet and back sheet to define an array of chambers. A set of holes or necks, usually one per chamber, penetrates the face sheet to establish communication between the chamber and the fluid stream. Each chamber and its associated neck is a Helmholtz resonator tuned (i.e. designed) to attenuate a narrow bandwidth of noise frequencies depending on the area and length of the neck, the volume of the chamber, and the local speed of sound. The liner is positioned along the duct wall with the face sheet extending approximately parallel to the direction of fluid flow through the duct.
During operation, the array of resonators attenuates noise attributable to pressure disturbances in the fluid stream. The effectiveness of a resonator in attenuating noise at its design frequency range depends on its ability to admit the disturbance into the chamber, a property referred to as acoustic admittance. Alternatively, the inability of a resonator to receive a disturbance is referred to as acoustic impedance, a complex quantity whose real component is known as resistance.
Despite the many merits of Helmholtz resonator acoustic liners, various factors can degrade their acoustic admittance. For example, when the liner is used to line a fluid handling duct, the flowing fluid grazes past the inlets to the resonator necks and, in doing so, reduces the acoustic admittance of the resonators. This occurs because the grazing fluid produces a region of fluid recirculation inside the neck, which reduces the effective area of the neck, thereby decreasing the acoustic admittance.
Another factor that can degrade acoustic admittance is the flow of coolant through the resonator necks. The use of coolant is often necessary when the duct carries a stream of high temperature fluid, such as the combustion products that flow through a turbine engine exhaust system duct. Cooling is normally accomplished by introducing the coolant (usually relatively cool air) into the resonator chambers through the chamber walls or through the liner back sheet. The coolant then flows out of the chambers by way of the resonator necks. The coolant flowing through the necks reduces the acoustic admittance of the resonator. The loss of acoustic admittance becomes more severe with increasing coolant Mach number.
Specific applications for acoustic liners and specific constraints imposed on their design can present additional challenges. For example, if the need to cool an acoustic liner was not anticipated during the early stages of product design, it can be challenging to retrofit a cooled liner into the product without adversely affecting other attributes of the product. And, irrespective of whether the need for an acoustic liner was appreciated early in product design, the severe space constraints faced by the designer of a gas turbine engines make it inherently difficult to design a liner that attenuates low frequency noise. This is because the design frequency of a Helmholtz is inversely proportional to chamber volume. Hence, large volumes (large amounts of space) are required to attenuate low frequency noise.